1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus having the zoom lens, and more particularly to a zoom lens suited for a film still camera, a video camera, a digital still camera or the like, which has three lens units in which a lens unit of negative refractive power leads, and which has the entirety of a lens system thereof reduced in size by appropriately setting the lens construction of the respective lens units.
2. Description of Related Art
In recent years, with the advancement of high performance of an image pickup apparatus (camera), such as a video camera or a digital still camera, using a solid-state image sensor, a zoom lens having a large aperture ratio including a wide angle of view is desired for the purpose of being used for an optical system of such an image pickup apparatus. Since, in such an image pickup apparatus, a variety of optical members, including a low-pass filter, a color correction filter, etc., are disposed between the rearmost portion of the zoom lens and the image sensor, a lens system having a relatively long back focal distance is required for the optical system. In addition, in the case of a color camera using an image sensor for color images, a zoom lens excellent in telecentricity on the image side is desired for an optical system of the color camera so as to prevent color shading.
Heretofore, there have been proposed a variety of wide-angle two-unit zoom lenses of the so-called short zoom type each of which is composed of a first lens unit of negative refractive power and a second lens unit of positive refractive power, the separation between the first lens unit and the second lens unit being varied to effect the variation of magnification. In such an optical system of the short zoom type, the variation of magnification is effected by moving the second lens unit of positive refractive power, and the compensation for the shift of an image point due to the variation of magnification is effected by moving the first lens unit of negative refractive power.
In such a lens construction composed of two lens units, the zoom magnification thereof is 2× or thereabout. Further, in order to make the entirety of a lens system in a compact form while having a high variable magnification ratio greater than 2×, there have been proposed, for example, in Japanese Patent Publication No. Hei 7-3507 (corresponding to U.S. Pat. No. 4,810,072), Japanese Patent Publication No. Hei 6-40170 (corresponding to U.S. Pat. No. 4,647,160), etc., the so-called three-unit zoom lenses in each of which a third lens unit of negative or positive refractive power is disposed on the image side of the two-unit zoom lens so as to correct the various aberrations occurring due to the high variable magnification.
Further, in U.S. Pat. No. 4,828,372 and U.S. Pat. No. 5,262,897, there is disclosed a three-unit zoom lens in which the second lens unit is composed of six lens elements, as a whole, including two cemented lenses, thereby attaining the high variable magnification of 3× or more.
Three-unit zoom lenses satisfying both the back focal distance and the telecentric characteristic have been proposed in, for example, Japanese Laid-Open Patent Application No. Sho 63-135913 (corresponding to U.S. Pat. No. 4,838,666), Japanese Laid-Open Patent Application No. Hei 7-261083, etc. In addition, in Japanese Laid-Open Patent Application No. Hei 3-288113 (corresponding to U.S. Pat. No. 5,270,863), there is disclosed a three-unit zoom lens in which a first lens unit of negative refractive power is fixed and a second lens unit of positive refractive power and a third lens unit of positive refractive power are moved to effect the variation of magnification. However, in these zoom lenses, there are such drawbacks that the number of constituent lens elements of each lens unit is relatively large, the total length of the lens system is great, and the production cost is high.
Further, in recent years, there has been widely used the so-called barrel-retractable zoom lens in which, in order to make the compactness of a camera and the high magnification of a lens system compatible with each other, the separation between the respective adjacent lens units at the time of nonuse of the camera is reduced up to the separation different from that at the time of use of the camera, thereby lessening the amount of protrusion of the zoom lens from the camera body. However, in a case where, as in the conventional zoom lenses, the number of constituent lens elements of each lens unit is large and, as a result, the length of each lens unit on the optical axis is great, or in a case where the amount of movement of each lens unit during zooming and during focusing is large and the total lens length is, therefore, great, it is sometimes impossible to attain the desired length of the zoom lens as retracted.
Further, in the zoom lens disclosed in Japanese Laid-Open Patent Application No. Hei 7-261083, a convex lens (positive lens) is disposed on the most object side of the first lens unit of negative refractive power, so that there is such a drawback that an increase of the outer diameter of the zoom lens when made to have a wide angle is inevitable. In addition, in this zoom lens, since the focusing onto a close object is effected by moving the first lens unit of negative refractive power, there is such a drawback that the construction of a lens mounting mechanism is complicated in combination with the movement for zooming.
Further, in U.S. Pat. No. 4,999,007, there is disclosed a three-unit zoom lens in which each of the first lens unit and the second lens unit is composed of a single lens. However, in this zoom lens, the total lens length at the wide-angle end is relatively great, and, because the distance between the first lens unit and the stop at the wide-angle end is large, the height of incidence of an off-axial ray of light is large to increase the diameter of a lens element of the first lens unit. Therefore, there is such a drawback that the entirety of a lens system becomes large.